Pull out the trigger assembly like you would to clean it. And then just put it back toghter without the internal magazine in it. Sometimes you will have to shave a little on the stock to make the magazines work.
If you are not familar with pulling the trigger group out.
1) Clear the weapon.
2) Lock the bolt open.
3) Make sure you put the safety on. Or you won't be able to push the pin in.
4) Look just behind the trigger assembly and you will see a pin.
5) Push in on it with a punch while pulling the trigger assembly straight up.
6) Pull the assembly out and set it aside now pull out the magazine.
7) Replace everything but the internal magazine, if you are wanting to use detachable magazines.

I think I went over everything. It's all memory to me when I sit in front of the gun and tear into it.